1. Field
This invention comprises a method and apparatus to reduce the wastewater treatment plant footprint of facilities employing rapid sludge chemical dewatering technology to produce a disinfected treated recovered wastewater.
2. State of the Art
Various sewage treatment methods and plants are known. Most large municipal systems employ a series of settling ponds sequentially concentrating the solids contained in wastewater either with or without polymers for separation from liquids via mechanical separation means, such as belt presses. To produce a clean effluent that can be safely discharged to watercourses, wastewater treatment operations use three or four distinct stages of treatment to remove harmful contaminants; according to the United Nations Environmental Programme Division of Technology, Industry, and Economics Newsletter and Technical Publications Freshwater Management Series No. 1, “Bio-solids Management: An Environmentally Sound Approach for Managing Sewage Treatment Plant Sludge” stating: “Each of these stages mimics and accelerates processes that occur in nature.
Preliminary wastewater treatment usually involves gravity sedimentation of screened wastewater to remove settled solids. Half of the solids suspended in wastewater are removed through primary treatment. The residual material from this process is a concentrated suspension called primary sludge, subsequently undergoing additional treatment to become bio-solids.
Secondary wastewater treatment is accomplished through a biological process, removing biodegradable material. This treatment process uses microorganisms to consume dissolved and suspended organic matter, producing carbon dioxide and other by-products. The organic matter benefits by providing nutrients needed to sustain the communities of microorganisms. As microorganisms feed, their density increases and they settle to the bottom of processing tanks, separated from the clarified water as a concentrated suspension called secondary sludge, biological sludge, waste activated sludge, or trickling filter humus.
Tertiary or advanced treatment is used when extremely high-quality effluent is required, including direct discharge to a drinking water source. The solid residual collected through tertiary treatment consists mainly of chemicals added to clean the final effluent, which are reclaimed before discharge, and therefore not incorporated into bio-solids. Tertiary or advanced treatment does not reduce the treated wastewater brine content, requiring energy intensive Quaternary brine treatment removal using reverse osmosis and distillation, and other methods.
Combined primary and secondary solids comprise the majority of material used at municipal plants for bio-solids production. Careful management throughout the entire treatment process allows plant operators to control the solids content, nutrient value and other constituents of bio-solids.
The Municipal Sludge-to-Bio-Solids Treatment Process
Three important factors must be addressed through further processing before this material can be utilized: (1) pathogen levels, (2) presence of potentially harmful industrial contaminants, and pharmaceuticals and personal care products, and (3) water content.
The principal process employed to convert municipal sludge into bio-solids is called stabilization. Stabilization accelerates the biodegradation of organic compounds, reduces the microbial population including pathogens, and renders the material microbiologically safe for agricultural use. Biological stabilization uses aerobic or anaerobic treatment to reduce the organic content of solids through controlled biodegradation. Chemical stabilization does not reduce the quantity of biodegradable organic matter in solids, but creates process conditions inhibiting microorganisms, thereby slowing the degradation of organic materials and reducing odors. The most common chemical stabilization procedure is to elevate the pH level of the solids using lime or other alkaline materials. Thermal drying and composting can be used to stabilize bio-solids. Full pasteurization of bio-solids is not needed when the primary use is cropland application. Any potential risk to human health due to exposure to pathogens is eliminated through proper application procedures and in-situ microbial decomposition.
The presence of contaminants in the sludge or bio-solids arising from industrial discharges is a more challenging problem and may be the deciding factor in determining the choice of a utilization disposal option. Put simply, many industries have habitually used the sewer system as a convenient and low-cost way to discharge hazardous wastes. The contaminants accumulate in the biomass and sludge, and can render the material unfit for any beneficial use. The most common options used for disposal of this contaminated material are landfill or incinerations. The cost is usually borne by the municipality rather than the hazardous waste generator. Bio-solids utilization is a good, environmentally sustainable option when the wastewater is from municipal sources only, or when a fully enforced industrial pre-treatment and discharge control system is in place. The decision to select an environmentally sustainable approach to bio-solids management can be used very effectively to review and correct polluting practices up-stream that should not be taking place.
The final concern is the water content of the bio-solids product. Primary and secondary sludge generally contain no more than four percent solids, and the storage and transportation costs of this semi-liquid material limit the application to nearby farmland. Processes to remove water from solids, therefore, are common in bio-solids production. The simplest method for removing water is gravity thickening, involving concentration by simple sedimentation. Allowing sufficient time for solids to settle in tanks can increase suspended solids concentration to five or six percent. Thickening can include flotation processes, gravity drainage belts, perforated rotating drums, and centrifuges. Nothing is added to bio-solids during the gravity thickening processes.
Dewatering is another standard method of water removal in bio-solids production. Simple dewatering involves containment of wastewater solids in drying beds or lagoons, where gravity, drainage, and evaporation remove moisture. More often, dewatering involves mechanical equipment including filter presses, vacuum filters, and centrifuges. Mechanically dewatered solids typically contain between 20% and 45% solids. Finally, drying processes can be used to remove even larger volumes of water from bio-solids. Thermal drying with direct or indirect dryers followed by polarization can remove virtually all water and stabilize bio-solids to the point of full compliance with any regulatory requirement. This method is used where a viable commercial market exists for the pelletized product.
Thus a particular wastewater treatment facility design is highly dependent upon the wastewater inflows and sludge composition and the discharge and treatment permitting restrictions and plant objectives. Oftentimes these plant designs employ thermophilic and other digestion processes to decompose the sludge as part of the separation process. For example, Haase, U.S. Pat. No. 5,906,750 issued May 25, 1999 discloses a method for dewatering of sludge previously digested by a thermophilic digestion process employing polymers. The polymers are extremely hydrophilic as they agglomerate fine particles for separation from the wastewater in the belt presses. This gelatinous mechanically separated mass is then usually land filled or admixed with other fuels for burning, and may contain significant pathogens and heavy metals. Once deposited and covered, these landfills do not breakdown rapidly. They comprise large deposits of unstable gelatinous soil, which acts as a breeding ground for pathogens. If these separated solids are treated with chlorine for pathogen kill, chlorinated carcinogens often result, creating a different environmental hazard.
The mechanically separated gray water by-product is usually not treated and is then used for agricultural application, or dumped into a body of water for dilution. If treated with chlorine to kill pathogens before land application or dumping, its usage for agricultural purposes is less than ideal as any residual chlorine acts as an herbicide.
In addition, mechanical sludge separation typically requires a large series of settling ponds with wastewater residence times therein typically from 24 to 48 hours, depending upon the weather and nature of the sludge processed. Typically, landfill and polymer costs represent approximately 30 percent of wastewater treatment costs. This long dwell time results in further concentrations of the brines.
Other mechanical filtration methods provide sludge separation, but require continual unplugging of the filters; generating significant ongoing costs of filter replacement and declining effectiveness as the filter becomes plugged with the separated solids.
As long as a mechanical sewage separation plant does not have to be moved and operates within its environmental discharge and landfill permit constraints, it can be used as a low operating and maintenance cost effective sewage disposal method. However, it is a technique requiring significant upfront capital investment, a large dewatering footprint, and may result in long term environmental clean-up and remediation costs. As urban populations being served grow and landfill costs increase, these plants seldom meet permitting constraints without significant upgrades in design, particularly with respect to pathogen gray water discharge and the negative impacts caused by mountains of gelatinous solids.
Other chemical wastewater treatment methods employ chemical agglomeration and disposal methods, such as Adams et al., U.S. Pat. No. 4,340,489 issued Jul. 20, 1982 wherein wastewater is treated with sufficient sulfurous acid to effectuate disinfection—usually approximately 5 to 100 mg/L free SO2 at pH 2.
Polymers and other separation methods are then employed to remove the solids. Reynolds et. al., U.S. Pat. No. 4,304,673 issued Dec. 08, 1981 is another wastewater treatment process employing chemicals to disinfect sewage sludge continuously in a similar manner as Adams et al. Rasmussen, U.S. Pat. No. 4,765,911 issued Aug. 23, 1988 is another two-stage chemical treatment process for treating aerobic or anaerobic sewage sludge. These chemical wastewater treatment methods are not package systems, not moveable as needed to accommodate the needs of a community, particularly in riparian areas subject to flooding, rely heavily on the use of polymers, and they do not address the issues of BOD's and ammonia in treated wastewater or brine disposal methods.
Theodore, U.S. Pat. No. 7,416,668 issued Aug. 26, 2008 discloses a wastewater chemical/biological treatment plant recovery apparatus and method employing sulfur dioxide for disinfection. Harmon et al., U.S. Pat. No. 7,455,773 issued Nov. 25, 2008 also employs sulfur dioxide for disinfection and dewatering and lime for pH adjustment. Theodore, U.S. Pat. No. 7,429,329 issued Sep. 30, 2008 also sulfur dioxide for chemical and mechanical dewatering. Theodore, U.S. Pat. No. 7,563,372 issued Jul. 21, 2009 discloses a package dewatering wastewater treatment method employing sulfur dioxide chemical dewatering in conjunction with mechanical agglomeration and disposal methods. Harmon et al., U.S. Pat. No. 7,566,400 issued Jul. 28, 2009 also employs sulfur dioxide as part of a chemical/biological treatment method and apparatus. These patents held by Earth Renaissance Technologies, Inc. use large concentrations of sulfur dioxide for rapid disinfection where it is desirable to reduce the dwell time to reduce tankage sizing and treatment plant footprint. This smaller footprint design then requires large amounts of lime or other chemicals for pH adjustment to neutralize excess sulfur dioxide; thereby increasing operating costs. Alternatively, lower concentrations of sulfur dioxide can be used with larger tanks to provide longer dwell times, thus increasing the treatment plant's capital costs and footprint, but lowering the amount of lime or other chemicals for pH adjustment and sulfur dioxide neutralization.
Thus, there remains a need for a method and apparatus to reduce the amount of sulfur dioxide for chemical rapid dewatering and disinfection and still provide a small treatment plant footprint that can easily retrofitted into existing wastewater treatment facilities to treat chemically and recover wastewater solids and liquids for subsequent environmental biological usage and polishing. The method and apparatus described below provides such an invention.